Engine systems are known which operate the engine with lean combustion, or a lean air/fuel ratio, to improve fuel economy. To accommodate lean burn conditions, emission control devices, such as nitrous oxide (NOx) traps, are used to adsorb nitrous oxide emissions produced during lean operation. Adsorbed nitrous oxide is periodically purged by operating the engine with rich combustion, or a rich air/fuel ratio.
During normal lean and rich operation, sulfur contained in the fuel can become trapped in the emission control device. This gradually degrades the emission device capacity for storing nitrous oxide, as well as the device efficiency. To counteract the sulfur effect, various sulfur decontamination methods are available.
One method for sulfur decontamination requires elevating the emission control device temperature to a predetermined value. Then, additional fuel is injected while the catalyst is at this elevated temperature to reduce the sulfur stored in the device. The temperature of the device is raised by operating some of the cylinders lean and some of the cylinders rich. When the lean and rich exhaust gasses meet in the device, exothermic reactions takes place, thereby releasing heat to increase the device temperature. The lean and rich exhaust gases are kept at certain desired lean and rich air/fuel ratios to maintain the average air/fuel ratio of the mixed exhaust gases at a desired air/fuel ratio. The desired lean and rich air/fuel ratios are determined in table look-up fashion with various correction factors. An exhaust gas air/fuel ratio sensor is relied upon to correct the desired lean and rich air/fuel ratios for control errors in the correction factors. Such a method is described in U.S. Pat. No. 5,657,625.
The inventors herein have recognized a disadvantage with the above approach. When the desired lean and rich air/fuel ratios are adjusted to control trap temperature, poor control is achieved. In particular, when the trap is at a low temperature, a large difference between the rich and lean air/fuel ratios is desired to rapidly increase temperature. However, when the trap is at a low temperature and the air/fuel ratio difference is increased, an initial drop in temperature is experienced because it takes a certain amount of time for the exothermic reaction to begin. Thus, the trap temperature can drop below the light off temperature. At this point, the temperature continues to drop since the exothermic reaction is no longer sustainable due to the trap being below the light off temperature.
Another disadvantage encountered when using the above approach is that if the air/fuel ratio difference between the rich and lean cylinders is made too large, the trap temperature can drop even when well above the light off temperature. This is because the additional exothermic heat from the air/fuel ratio difference is not large enough to counteract the lower exhaust temperature caused by operating lean of, or rich of, stoichiometry.